The present invention relates to electrical resistance corrosion monitors for detecting loss of material due to corrosion and/or erosion caused by the interaction of that material with its environment. More specifically, the present invention relates to probe devices for electrical corrosion monitors.
Corrosion monitors are commonly used for detecting and monitoring loss of material due to corrosion and/or erosion caused by the interaction between the material and the environment in contact with the material. Corrosion monitors generally detect the loss of material by an electrical resistance method. Such corrosion monitors typically comprise a probe device that has an element comprising a material that exhibits similar erosion/corrosion susceptibility as the material to be monitored, for example the metal material of the interior surface of steel pipes used in gas and oil pipelines. The element is situated in the same environment, e.g. the pipeline fluid, that is in contact with the metal material, e.g. steel pipe. The resistance of the element increases due to reduction of the cross-sectional area of the element caused by the corrosion and/or erosion of the element by the environment in which the element and metal material are situated. The increase in resistance of the element provides an indication of the corrosion and erosion of the metal material.
Resistance of the element of the metal material in corrosion monitors is commonly monitored by supplying a current through the probe device holding the exposed element and a reference element, where the reference element is protected from and inaccessible by the fluid and in series with the exposed element. The voltages across both elements are measured and the resistance ratio of the exposed element resistance to the reference element resistance is then calculated. The change of resistance ratio is representative of the loss of material of the exposed element. Of course, the sensitivity of the corrosion monitor is dependent on both the current supplied to the elements and the resistance of the elements. Accordingly, the larger the current and/or the larger the resistance values of the elements, the greater the sensitivity of the corrosion monitor.
However, the sensitivity of the corrosion monitor is limited by various factors. For instance, the sensitivity of the corrosion monitor is dependent on the maximum current and the maximum resistance of the element. The environment in which the element is situated is often potentially explosive, such as in gas and oil pipelines. The maximum current in such environments is limited to intrinsic safety requirements. Typically the maximum current permissible is less than 100 mA in such environments and the typical resistance of the element is usually in the range of 1 to 10 mxcexa9. Higher element resistances, thus higher sensitivities, are attainable with elements having smaller cross-sectional areas, e.g. less than 0.5 mm thick, although, the useful life of such an exposed element is reduced. Accordingly, the sensitivity of corrosion monitors is limited by the relatively small measured resistive voltages which are usually in the range of 10 to 100 xcexcV. Disturbances such as noise and dc offsets occurring in the electronic circuitry of the corrosion monitor and thermoelectric voltages and electromagnetic noise occurring in the leads between the electronic circuitry and the probe make accurate high resolution measurements of such small voltages difficult.
Additionally, changes in temperature in the environment in which the element is situated changes the resistance of the element. For example, the resistance of steel may change by 0.4% per xc2x0 C. Accordingly, in electrical resistance corrosion monitors configured with an exposed element and a reference element, changes in fluid temperatures significantly limits the accuracy and sensitivity of the monitor if the temperature of the exposed:element and reference element differ.
In the prior art designs, different values of temperature coefficients of resistivities of the elements limits the performance of the corrosion monitor. Even if the temperature coefficient of resistivity values differ as little as 10 ppm xc2x0 C.xe2x88x921, a change in probe temperature of 50xc2x0 C. causes a change in the resistance ratio by 500 ppm, giving a false indication of corrosion. Hence, the accuracy and sensitivity of such prior art electrical resistance corrosion monitors is limited to detect corrosion in days or months. Such devices are not effective for many corrosion management applications such as in controlling and detecting the addition of corrosion inhibitors introduced in pipeline fluids.
Attempts have been made to increase the sensitivity and improve the resolution of measurements to the changes in the resistance of the element due to corrosion and/or erosion of the element in electrical resistance corrosion monitors. However, the sensitivity in such corrosion monitors is typically limited to several days before corrosion can be measured. For example, European Patent Application No. 84303370.5 discloses a flush mounting electrical resistance corrosion probe that has a sample element and a reference element where the reference element is shielded from the corrosive fluid environment by the sample element and an insulating material plug of the probe. Although the reference element is responsive to the temperature conditions of the environment in such a configured probe, the reference element and the sample element may not be at substantially identical temperatures during corrosion monitoring. Since the reference element is protected from the fluid environment by the sample element and electrically insulating material of the plug that has low thermal conductivity, the temperature and resistance of the reference element changes more slowly than the temperature of the sample element. Even slow temperature changes producing small temperature differences between the sample element and the reference element may produce significant changes in the resistance ratio. Additionally, the elements are formed in a manner that the values of temperature coefficients of resistivity for each element may differ. Consequently, these factors limit the sensitivity of such a configured probe and can yield spurious indications of corrosion when the temperature of the fluid environment is changing or even when the temperature of the fluid is constant as the temperature in the probe may differ from the fluid environment temperature.
Therefore, there is a need for an electrical resistance corrosion monitor with a greater sensitivity to accurately measure at a higher resolution, for example in minutes or hours, the corrosion and/or erosion of a material in a corrosive/erosive environment where the environment temperature may be fluctuating.
An object of the present invention is to provide an apparatus with improved sensitivity for monitoring change in the thickness of an element.
The present invention provides a probe device for use in an apparatus for monitoring changes in the resistance of an element caused by a fluid at the surface thereof, said probe device comprising a first element having a surface that is directly accessible. by a fluid, a second element electrically connected in series with said first element and forming therewith a path For the passage of electrical current to flow therethrough, wherein said first element and said second element are formed from the same piece of material, said piece of material being divided along an elongate slot to form a boundary of said first element on one side of said slot and a boundary of said second element on the other side of said slot.